How to mix concrete

The ratios everybody uses, and what they are not

Three mixes cover almost all site work, and all three are quoted as parts of cement to sand to coarse aggregate, measured by volume:

• 1:2:4. The general one. Paths, bases, small slabs, setting posts.
• 1:1.5:3. Richer in cement, for where more strength is wanted.
• 1:3:6. Lean. Mass fill, blinding, backfilling a trench.

In Britain those often collapse into cement and ballast, which is sand and stone already combined, at roughly 1:5 or 1:6. It is one material to buy and one less thing to get wrong.

Now the important part. Those numbers are trade convention. They are not a specification, nothing is certifying them, and they carry no strength class. The concrete you can order does carry one, which is what the concrete mix reference is about, and that page will tell you why FND exists and what PAV is doing. Anything holding a building up is ordered.

Water decides the strength, and it is the ingredient people are most generous with

Every other part of mixing concrete is forgiving. This one is not, and it runs against instinct, because the mix that feels easiest to work with is the weak one.

Cement consumes a fixed amount of water chemically as it cures. Anything you add beyond that is simply sitting in the mix taking up room. It makes the concrete flow, it makes it easy to place, and then over the following weeks it leaves. What it leaves behind is the space it was occupying.

Which produces the single most useful habit on this page: add water in stages and stop early. You cannot take it back out. A mix that has gone sloppy is not recoverable by adding more cement, because you have changed the proportion of everything at once.

Wet sand matters here more than people expect. Sand off an uncovered heap after two days of rain carries a lot of water into the mix before you have added any, so the same recipe wants noticeably less from the bucket than it did last week.

A shovel is not a unit

Ratios measured in shovelfuls drift, because a shovel of dry cement and a shovel of damp ballast are not the same quantity and no two people load one the same way. Over a morning that drift is the difference between the first barrow and the last.

Use one bucket for everything and count. A builder's bucket for each part, the same bucket, filled the same way. It costs nothing, it makes the mix repeatable, and repeatability between batches is worth more on a slab than hitting any particular ratio exactly. A slab of consistent ordinary concrete behaves better than a slab that is strong in patches.

Dry first, then water, and never top it up afterwards

The order is fixed and skipping the first step is the commonest reason hand-mixed concrete comes out streaky.

• Combine the dry materials until the colour is even. Cement, sand and aggregate, turned over until there is no grey marbling left in it. In a drum, a couple of minutes.
• Add roughly half the water you expect to need. Then mix again before judging.
• Bring it up in small amounts. Stop when it holds the shape of the shovel drawn through it.
• Place it, then leave it alone. Concrete that has begun to stiffen is finished being adjusted.

That last one has a name. Retempering is adding water to a batch that has started to go off to make it workable again, and it does real damage: the cement has already begun building structure, the water breaks it up, and the extra water then leaves its own voids on top. The result handles like concrete and is meaningfully weaker than what you started with. On a warm day the answer is smaller batches, which is unpopular and correct.

Checking it without a lab

Draw the shovel through the mix and look at the furrow. A mix at about the right consistency holds the ridges and the sides stand up. If the furrow closes and levels itself out, there is too much water in it. If the ridges crumble and the surface looks dusty, there is not enough, and that is the safer of the two errors to have made because you can still add.

The other check is the barrow. Concrete that slides around as a liquid when you move it has been mixed too wet. Concrete that stays as a heap and moves as one thing is about right.

When to stop mixing and phone a supplier

There is a quantity above which mixing on site stops being sensible, and it arrives sooner than most people plan for. Somewhere around a cubic metre, hand mixing has become a full day of hard labour and the consistency across that many batches is genuinely difficult to hold.

The concrete calculator works out which side of that line you are on, including the minimum load trap that makes small deliveries expensive, and the mix reference covers what to ask for once you have decided to order. If there is steel going into it, the rebar schedule turns the bars into a list a supplier can cut from.

And the line that has not moved: a footing, a raft or anything carrying load is not a site-mix job. Not because you could not physically do it, but because the specification for it came from somebody who looked at the ground, and a bucket ratio has no way to meet a specification it does not know about.